The present invention relates to a control apparatus for a hybrid vehicle which has a plurality of drive modes and changes the drive mode according to predetermined conditions.
In recent years, there have been proposed and developed various control apparatuses for the hybrid vehicle in which a first engagement element to connect/disconnect an engine and a motor and a second engagement element to connect/disconnect the motor and driving wheels are provided. The hybrid vehicle has, as drive modes, a motor-use drive mode (hereinafter called “EV drive mode”) in which the vehicle travels with only the motor being a power source and an engine-use drive mode (hereinafter called “HEV drive mode”) in which the vehicle travels with the engine included as the power source. In such control apparatuses, for example, Japanese Patent Provisional Publication No. 2005-221073 (hereinafter is referred to as “JP2005-221073”) discloses a control apparatus of the hybrid vehicle which increases gas mileage by changing the drive mode in accordance with a vehicle condition and a traveling condition.
In such hybrid vehicles, when a required driving force by a driver is high, the vehicle could start using both driving forces of the engine and the motor to meet the driver's required driving force at the vehicle start. In this case, since there is no element such as a torque converter which serves to absorb input rotation speed and change of the rotation speed, when the vehicle starts with the first and second engagement elements completely engaged, a vehicle speed is determined according to an engine rotation speed. On the other hand, regarding the engine, there is a lower limit value of an idle rotation speed for maintaining a self-rotation of the engine. Under a condition in which an idle-up control by engine warming-up etc. is being carried out, this lower limit value of the idle rotation speed becomes even higher.
Therefore, at the vehicle start when the driver's required driving force is high, it is required to fulfill a rotation absorption function while meeting the required driving force. Also, during such an extremely low speed travel that the engine rotation speed falls below the lower limit value of the idle rotation speed, it is required to fulfill the rotation absorption function while meeting a demand for engine stall avoidance. Thus, by performing a slip-control of the second engagement element installed on a driveline between the motor and the driving wheels while maintaining the engagement of the first engagement element, the vehicle start and travel using the engine also the extremely low speed travel can be realized. This drive mode is called “WSC (Wet Start Clutch) drive mode”.